In the field of wireless communication systems, HSDPA (High Speed Downlink Packet Access) has been proposed, which allows a plurality of communication terminals to share a high-rate and large-capacity downlink channel to perform high-rate packet transmission via the downlink channel. In addition, these days, a technique for speeding up the packet transmission rate on an uplink channel has been under study (such a technique is referred to as Fast-UL (Fast—Uplink) herein). In HSDPA, a plurality of channels is used, including HS-PDSCH (High Speed—Physical Downlink Shared CHannel), A-DPCH (Associated—Dedicated Physical CHannel), HS-DPCCH (High Speed Dedicated Physical Control CHannel), etc. Likewise, Fast-UL is expected to involve a plurality of channels such as HS-PUSCH (High Speed—Physical Uplink Shared CHannel), A-DPCH, HS-DPCCH, and so on.
HS-PDSCH is a downlink shared channel used for packet transmission. HS-PUSCH is an uplink shared channel used for packet transmission. Accompanying a shared channel, A-DPCH, which is a dedicated associated channel on uplink and downlink, transports pilot signals, TPC (Transmission Power Control) commands, control signals for keeping a communication link, etc. HS-DPCCH is a dedicated control channel in uplink and downlink, over which signals for controlling a shared channel such an ACK signal or a NACK signal, and a CQI (Channel Quality Indicator) signal, etc. are transmitted. Incidentally, an ACK signal is a signal indicating that a high-rate packet which had been transmitted from a base station or from a communication terminal was correctly demodulated at a communication terminal or at a base station, whereas a NACK signal is a signal indicating that a high-rate packet which had been transmitted from a base station or from a communication terminal was erroneously demodulated at a communication terminal or at a base station. Additionally, CQI is a signal which is generated based on channel quality, indicating a combination of, for example, a packet modulation scheme, a block size, a transmission power adjustment value, and so forth. In HSDPA, a communication terminal notifies its communicating party about a packet modulation scheme, a block size, a transmission power adjustment value, etc., as desired by the communication terminal, by using such a CQI. Although CQI is a signal which is generated based on channel quality also under Fast-UL, its specific contents have not been fixed yet.
Incidentally, in Fast-UL, channels are provided on both uplink and downlink for both A-DPCH and HS-DPCCH, where CQI is transmitted via an uplink HS-DPCCH whereas an ACK/NACK signal is transmitted via a downlink HS-DPCCH. In contrast, according to HSDPA, though channels are provided on both uplink and downlink for A-DPCH, an uplink channel only is provided for HS-DPCCH, where CQI and an ACK/NACK signal is transmitted on an uplink HS-DPCCH. Soft handover (SHO) is employed in A-DPCH. In contrast, HS-PDSCH, HS-PUSCH, and HS-DPCCH are subjected to hard handover (HHO), which means that HS-PDSCH, HS-PUSCH, and HS-DPCCH are always connected to a single base station only. Moreover, the timing for executing HHO on HS-PDSCH and HS-PUSCH is the same as the HHO timing of HS-DPCCH.
With reference to FIG. 1 through FIG. 3, transmission power control for HS-DPCCH is explained below, taking an example of Fast-UL. FIG. 1 illustrates a situation where A-DPCH is not under SHO, while FIG. 2 and FIG. 3 show a state in which A-DPCH is under SHO. Here, the state where A-DPCH is not under SHO means a situation in which a communication terminal has an A-DPCH connection to a single base station only, whereas the state where A-DPCH is under SHO means a situation in which a communication terminal has A-DPCH connections to a plurality of base stations.
As illustrated in FIG. 1, the transmission power of A-DPCH is controlled under a well-known practice of a closed loop transmission power control in accordance with TPC commands so that the reception SIR of A-DPCH is kept at the target SIR thereof. On the other hand, as for HS-DPCCH, transmission power control is performed in accordance with A-DPCH TPC commands in a similar manner as done for A-DPCH. This ensures that the received SIR of HS-DPCCH meets the required SIR when A-DPCH is not under SHO.
As a communication terminal moves toward base station 2 from base station 1, the communication terminal establishes connections to base station 2 as well as base station 1, which puts A-DPCH in SHO conditions. Then, the transmission power control of HS-DPCCH, to which HHO applies, is performed in the following way when A-DPCH is under SHO.
First, transmission power control of uplink HS-DPCCH is explained with reference to FIG. 2. When A-DPCH enters into SHO, both of base station 1 and base station 2 receives A-DPCH signals transmitted from the communication terminal. Base station 1 generates a TPC command so as to make the reception SIR at BS 1 approaches the target SIR, and transmits the generated command to the communication terminal. Base station 2 generates a TPC command so as to make the reception SIR at BS 2 approaches the target SIR, and transmits the generated command to the communication terminal. The communication terminal increases transmission power of A-DPCH if all of a plurality of the received TPC commands is TPC commands for instructing to increase transmission power, while the communication terminal decreases transmission power of A-DPCH if any one of the plurality of the received TPC commands are a TPC command for instructing to decrease transmission power. Accordingly, in a case where a TPC command for instruction of increasing transmission power is transmitted from base station 1 whereas another TPC command for instruction of decreasing transmission power is transmitted from base station 2, the communication terminal decreases the transmission power of an A-DPCH signal. Because the transmission power of HS-DPCCH is controlled in the same way as the transmission power of A-DPCH, as illustrated in FIG. 2, the transmission power of an HS-DPCCH signal is also decreased in accordance with the decreasing of the transmission power of an A-DPCH signal.
Here, regarding the uplink A-DPCH, an A-DPCH signal received at base station 1 and an A-DPCH signal received at base station 2 are subjected to selective combination at a control station. For this reason, even in a case where the transmission power of A-DPCH is decreased as described above, no particular problem arises because the SIR of the uplink A-DPCH meets the required SIR at the control station.
In contrast, as for HS-DPCCH to which HHO applies, a connection is made to either one of the base stations only even when A-DPCH is in SHO. For this reason, when the transmission power of the uplink HS-DPCCH is decreased in accordance with the decreasing of the transmission power of the uplink A-DPCH as described above, it is possible that the SIR of the uplink HS-DPCCH fails to meet the required SIR.
Next, transmission power control of downlink HS-DPCCH is explained with reference to FIG. 3. When A-DPCH enters into SHO, a communication terminal receives A-DPCH signals transmitted from both of base station 1 and base station 2. Combining the A-DPCH signal transmitted from base station 1 and the A-DPCH signal transmitted from base station 2, the communication terminal generates a TPC command in such a manner that the reception SIR of the combined signal approaches the target SIR. Then, the communication terminal transmits the identical TPC commands respectively to base station 1 and base station 2.
Here, even when the reception SIR falls short of the target SIR at the communication terminal only with the A-DPCH signal transmitted from base station 1, as illustrated in FIG. 3, a TPC command for instructing the decreasing of transmission power is transmitted if the reception SIR of the combined signal equals to or exceeds the target SIR. Because the transmission power of HS-DPCCH is controlled in the same way as the transmission power of A-DPCH, as illustrated in FIG. 3, at base station 1, the transmission power of an HS-DPCCH signal is also decreased in accordance with the decreasing of the transmission power of an A-DPCH signal as per the TPC command.
As for HS-DPCCH to which HHO applies, a connection is made to either one of the base stations only even when A-DPCH is in SHO. For this reason, when the transmission power of the downlink HS-DPCCH is decreased in accordance with the decreasing of the transmission power of the downlink A-DPCH as described above, at the communication terminal, it is possible that the SIR of the downlink HS-DPCCH fails to meet the required SIR.
It is noted that this problem arises similarly in HSDPA, too, not only in Fast-UL.